1. Field of the Invention
The invention relates to a directly mixing synchronous AM receiver comprising a synchronous demodulator coupled to an antenna input for synchronously demodulating an RF-AM reception carrier, and a phase-locked loop (PLL) incorporating a phase detection arrangement, a loop filter, a controllable oscillator which is coupled to the synchronous demodulator via an in-phase output and to the phase detection arrangement via a quadrature output.
2. Description of the Related Art
A directly mixing synchronous AM receiver of this type is known from U.S. Pat. No. 4,631,499 (PHN 11, 220).
The phase-locked loop of the directly mixing synchronous AM receiver is used for regenerating a local mixing carrier which is applied to the synchronous demodulator via the in-phase output of the controllable oscillator and which should accurately follow the phase of the RF-AM reception carrier for the purpose of a correct synchronous demodulation of this carrier.
However, in practice a phase error is produced between these two carriers due to different factors such as, for example, an unwanted parasitic DC offset of the phase control signal for which the above-mentioned prior art provides a solution. Another important factor is the field strength-dependent conversion gain in the phase detection arrangement so that the phase control signal in the loop not only depends on the phase difference between the two carriers applied to the phase detection arrangement, but also on the amplitude, i.e. field strength of the RF-AM reception signal.
The field strength dependence of the phase control signal can be reduced by means of an automatic gain control arrangement before or in the loop in such a way that a stable, accurate phase synchronism between the two carriers is obtained within a certain range of field strength variation or dynamic range of the receiver. A field strength indication is derived from the amplitude level, which is average over a given period of time, of the demodulated RF-AM reception carrier at the output of the synchronous demodulator and is applied as a DC gain control signal to the gain control arrangement. However, it has been found that the feasible dynamic range is comparatively small with such known automatic gain controls.